In a human spine the intervertebral disc acts as a stabilizer and as a mechanism for force distribution between the vertebral bodies. When the intervertebral disc is not present, the intervertebral space collapses causing pain, abnormal joint mechanics and premature development of arthritic changes.
Methods of treating injured or diseased discs have included chemical disintegration procedures and surgical excision, often followed by bony fusion to prevent spinal collapse or instability. With disc excision, no significant regeneration of disc material occurs. Replacement of an injured disc in an otherwise healthy spine may prevent arthritic changes and may stabilize the spinal segments. In diseased spines, replacement of the disc may reduce the progression of the disease process and may provide pain relief. Several methods have been developed for providing stability, via internal fixation, for the purpose of improving the probability, rate, and time associated with spinal fusion.
Methods of achieving internal fixation include the use of rods, plates and interbody cages. Examples of interbody cages are found in U.S. Pat. Nos. 4,961,740, 5,015,247 and 5,425,772. The cages or fusion devices described in these patents have threaded hollow bodies, which allow for the packing of autologous bone grafts. Typically, interbody cages are filled with bone graft material, either autograft or allograft, in order to enhance long term fixation of the interbody cage via bone attachment to the opposing vertebral end plates through the cage. Bone grafting is associated with a statistical chance of post-operative complications. The exact nature of these complications is generally related to the source of the graft material, but includes harvest site pain and immunological responses. Bone used as graft implants is often removed from another portion of a patient's body, which is called an autograft. A significant advantage of using a patient's own bone is the avoidance of tissue rejection, but harvesting bone also has its shortcomings. There is a risk to the patient in having a second surgical procedure (bone harvesting) performed at a secondary site which can lead to infection or additional pain to the patient. Further, the bone harvesting site is weakened by the removal of the bone. Other options for a bone graft source is bone removed from cadavers, called allograft, or from an animal called xenograft. While these kinds of bone grafts relieve the patient of having a secondary surgical site as a possible source of infection or pain, this option carries a high incidence of graft rejection and an increased risk of the transmission of communicable diseases. Further, xenograft and allograft material are more slowly incorporated into a patient's body than autograft material.
In these devices long term fixation is attempted via bone "throughgrowth," in which existing bone tissue replaces the graft material. However, due to the relative stiffness of the cage portion of these devices, it is extremely doubtful that the bone within the cage remains viable over the long term as this mechanically unloaded bone would be reabsorbed, leaving only fixation via the external threads on the cage.
Another spinal fusion device, described in U.S. Pat. No. 4,878,915, is a solid plug implant with roughened outer surfaces for receiving bone ingrowth in order to fuse the plugs in the intervertebral space. The roughened outer surface on this device allows for only attachment of the device through interdigitation of the bone with the roughened outer surface of the plug.
Accordingly, it is an object of this invention to provide a spinal fusion device for fusing adjacent vertebral bodies without the need to use bone graft material. Another object of the invention is to provide for internal fixation/stabilization of the vertebrae to be fused through the use of a solid threaded device which has indentations in its outer surface containing osteoconductive material that contacts the bony end plates of the vertebral bodies to be fused. Yet another object is to provide a spinal fusion device that does not require bone growth into a hollow center of the device in order to augment the threaded fixation mechanism on the outer surface of the fusion device.